1. You essentially only pay you transportation costs and for any souvenirs you may buy. During the time of your stay at the host farm, you have your lodgings and food covered.
2. You can learn the local language if you want. Whether a beginning or advanced student of the language, you will definitely add to your language skills if you put in the effort to do so. Hosts are most likely quite happy to have you practise speaking in their language as long as it does not interfere with their ability to explain tasks in a way you can understand.
3. You learn other skills - I learned to milk goats, to take goats and sheep to pasture with a sheep dog, to make one kind of cheese. Other skills that can be learned are the various aspects of permaculture, such as composting, building construction using local materials, cropping using natural means of weed and pest control, and more.
4. You are immersed in the local culture and you learn your host family's way of life: daily schedule, meal planning and eating patterns, social interactions, family ties, community dynamics.
5. Meet people from other parts of the world. There may be other wwoofers together with you and you thereby meet other interesting people of all ages who are wwoofing each for their own reasons. You hear about other places that may be worth visiting and you learn more about how to make the wwoofing experience a hassel-free enjoyable one.